Drilling mud is circulated in the borehole through the drill bit to facilitate the drilling process. The circulating mud takes away the cuttings formed by the drill bit and brings them to the surface, where they are separated from the drilling fluid and the drilling fluid is recycled. As the well is drilled deeper, the column of mud acting on the formation at the bottom of the wellbore grows longer, thereby exerting a greater pressure on the formation. In certain formations, the presence of pressure on the pay zone, even during drilling, can adversely affect the ultimate performance of the well when it is brought in, due to formation damage caused by the drilling medium invading virgin formation.
In the past, the techniques that have been employed to address this problem have involved an insertion of a tubular with gas injection behind the tubular in an annular space at the periphery of the wellbore. The gas is inserted at the surface and must pass to the bottom of the tubular in order to come up and around inside the tubular back to the surface. Thus, attempts to lighten the pressure exerted on the formation by the column of mud in the wellbore being drilled have been limited due to the inability to inject gas at any other point except close to the bottom of the wellbore. The density of the circulating mud changes during the drilling operation and, thus, the ability to only inject gas very near the drill bit at times does not permit proper control to maintain the balanced or underbalanced condition. Underbalanced drilling has been attempted using nitrified drilling fluid or other light drilling fluids. However, these techniques had problems.
Various designs of tubing valves used downhole are illustrated in U.S. Pat. Nos. 3,583,481; 4,602,684; 4,257,484; 3,407,830; and 3,398,760.
Accordingly, one of the objectives of the present invention is to provide access from a parasite annulus along the length of the annulus so that injection can occur at various depths. Another objective is to provide adjustability for each of the communication valves through from the parasite annulus to allow more control of the distribution of gas from the parasite annulus into the mud column for better controlled drilling conditions.